\documentclass{article}


%%-----------PGF/TIKZ
\usepackage{pgf}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes,shapes.arrows,positioning}

\usepackage{palatino}

% Use preview.sty to crop the page
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{pgfpicture}
\setlength\PreviewBorder{0pt}

\begin{document}

\begin{figure}
\begin{tikzpicture}
    [
    node distance=10mm and 10mm,
    scale=.8,
    block/.style ={
        rectangle,
        draw=black, 
        thick, 
        top color=gray!20, 
        bottom color=white,
        text width=5em, 
	    text centered,
    },
    ]

	\node(begin) [text width=12em,text centered] {Begin with last $m$ days of precipitation data (including today)};
    \node(subse) [block,below=of begin] {Subtract Seasonal Precip Component};
    \node(subtr) [block,below=of subse] {Subtract Average Precip Trend};
    \node(cor) [block,right=of subtr] {Correlate with Inflow Residual};
    \node(addtr) [block,right=of cor] {Add Average Inflow Trend};
    \node(addse) [block,above=of addtr] {Add Seasonal Inflow Component};
    \node(end) [text centered,text width=7em,above=of addse] {End With 24 Hour Inflow Forecast};
    
    \path[draw,double distance=1.2pt,-latex'] (begin) -- (subse);
    \path[draw,-stealth'] (subse) -- (subtr);
    \path[draw,-stealth'] (subtr) -- (cor);
    \path[draw,-stealth'] (cor) -- (addtr);
	\path[draw,-stealth'] (addtr) -- (addse);
	\path[draw,double distance=1.2pt,-latex'] (addse) -- (end);

	
\end{tikzpicture}
\end{figure}
\end{document}
